Intermittent A/C very cold and then warm

[rockola1971]

Attached gauges at shop for intermittent a/c problem. This is the same 2005 Tahoe I did the exhaust manifold clamps on recently. Drove to pick her up and was ice cold and 1/2 way there, started blowing warm till got to her house. Shut off. She got in. Drove back to my house and a/c worked the whole way.

At shop today.....Ambient temp at 84 degrees.....29 degrees out the vent..... low pressure at 25 psi and high pressure at 200 psi.....system working normal..

Found 2 history codes: B0408-Temperature Control 1 Circuit Malfunction & B0418-Temperature Control 2 Circuit.

That low pressure reading sounds like anything but normal at 84 degrees ambient. Id expect to see 45psi or so at that ambient temp. It appears your system is low on charge. What does the pipes look and feel like coming off the evaporator and drier under the hood with system running? Have alot of frost or ice?

 

[OR VietVet]

Piprs are very cold and slight wet. So cold that when touch them they surprise you.

 

[nonickatall]

Piprs are very cold and slight wet. So cold that when touch them they surprise you.

So that means your AC is working. A compressor does not work sometimes, sometime not. When they are defective, they are out of service. Except when you have intermediate problems with the magnetic clutch, which is rare from my experience.

So the issues left, could be the actuator,s or the HVAC. Because it is least work, I would swap the HVAC with a used one, example from your truck, just to see if it works. If not, change the temperature actuators. The one which makes noise and also the driver side, because the problem can come as well from the position detection which makes normally no noise.

And the driver side is easy to replace...

 

[Sir_Hiro]

That low pressure reading sounds like anything but normal at 84 degrees ambient. Id expect to see 45psi or so at that ambient temp. It appears your system is low on charge. What does the pipes look and feel like coming off the evaporator and drier under the hood with system running? Have alot of frost or ice?

From the GM Service Manual.
Your pressures seem a bit odd. Which can cause all sorts of weird problems.
I agree with @rockola1971 I would verify your pressures and charge first.

 

[nonickatall]

View attachment 465529
From the GM Service Manual.
Your pressures seem a bit odd. Which can cause all sorts of weird problems.
I agree with @rockola1971 I would verify your pressures and charge first.

What is wrong with the pressure?

He writes 25psi on low and 200psi on high side at 84°.

That's ok,from my view.

Our cars have a pressure switch for high and low side. So when you have less refrigerant in the system the low switch prevents engaging the compressor and if the ist too much refrigerant the high pressure switch disables the compressor.

What should odd happen? Can you explain?

I do AC service in my garage, I have AC service unit and the cooling system seems ok to me, following his description.

To me it seems more like an HVAC/Actuator Problem...

But I am curious what will be se solution to this case...

 

[OR VietVet]

I have driven it when the a/c is working. It blows COLD. Not cool, but COLD. I don't see a low charge problem.

Tomorrow morning, I am going to clear the codes and do a Re-Calibration and see what happens. She called me today and said that it blew cold all day. Again, COLD. @915_Tahoe did locate a NOS control head/moduleat a GMC dealer in NH.

 

[rockola1971]

What is wrong with the pressure?

He writes 25psi on low and 200psi on high side at 84°.

That's ok,from my view.

Our cars have a pressure switch for high and low side. So when you have less refrigerant in the system the low switch prevents engaging the compressor and if the ist too much refrigerant the high pressure switch disables the compressor.

What should odd happen? Can you explain?

I do AC service in my garage, I have AC service unit and the cooling system seems ok to me, following his description.

To me it seems more like an HVAC/Actuator Problem...

But I am curious what will be se solution to this case...

The low pressure cutoff switch actuates around 20PSI or so. OP isnt low enough for the low pressure cutoff to disable his compressor to protect the system. The system is blasting super cold air because the evaporator is running at below freezing temps and icing over. At 24.5 PSI the refrigerant is at 28 deg!
A system that is overcharged or undercharged will act similar because one or more sections will be either flooded or starved for refrigerant. The pressures wont be alike though.

 

[OR VietVet]

It has never been icing over when I see it. Also, codes related B0408 and B0418, IIRC, are pointing at the temp actuator but that makes sense with the hunting and rarcheting actions of thar actuator. She can make the cold/first start of the day and drive over for me to look at and the a/c will not turn on and if I jump the relay pins 30 to 87, it blows cold.

 

[rockola1971]

It has never been icing over when I see it. Also, codes related B0408 and B0418, IIRC, are pointing at the temp actuator but that makes sense with the hunting and rarcheting actions of thar actuator. She can make the cold/first start of the day and drive over for me to look at and the a/c will not turn on and if I jump the relay pins30 to 87, it blows cold.

You likely have to jump the relay because the low pressure cutoff is preventing the relay from being energized. You are almost 50% undercharged at 25 PSI.

 

[OR VietVet]

You likely have to jump the relay because the low pressure cutoff is preventing the relay from being energized. You are almost 50% undercharged at 25 PSI.

When it is acting up, I tried to jump the low pressure switch, at the accumulator, and it will not activate the clutch. I unplug the connector and use the same jump kit/set up I have for things like that.

 

[nonickatall]

My experience is, if you reach the point we're not enough refrigerant is in the system, the system will not work anymore.

It's absolutely not common that the system works and after a while stopps working by disengage the compressor. And then work again. And sometimes work without problems, when I understood that correctly...

@QR VietVet How did you measure that 25 PSI on low and 200 PSI on high side? With a AC service unit while engine running idle?

 

[OR VietVet]

I work as a part time service writer at my friend's shop in Junction City, Oregon. He is a specialist in so many fields and has done work on this Tahoe before. He used his Snap On gauges and reported to me what the readings were. No charge for the inspection. It was working when he inspected.

This morning she came in at 9 a.m. and I cleared the codes in the system. She did not use the a/c to get to shop. Was still in the low 60's. B0408 and B0418. I then did a re-calibrate on it.

Shut the engine off. Moved the manual temp controls up from bottom to about 3/4" up. Turned fan speed switch to 3. Removed the 10 amp HVAC/ECAS fuse, from the under hood fuse/relay box, for 1 minute. Put the fuse back in and then started the truck and let run for 2 minutes. Shut truck off. Let the truck sit for about 15-20 seconds and then restarted. Then something strange happened that had never happened before during this ordeal she has been having. Instead of having either all cold out of all four vents of the D/S and P/S, it blew cold on D/S and hot on P/S. No matter what I did by shutting off the system and shutting off engine, on a restart and set for a/c to work, it blew cold on D/S and warm on P/S.

Then I did the re-calibrate again and we got surprised again. Everything worked as is intended. Blew cold when commanded to and warm when commanded to and was same for both the D/S and the P/S. In other words, it acted like there was never a problem. She drove home and the a/c worked great the whole way. She is gonna monitor it during the day because she will be using the a/c today in 90 degree weather.

 

[nonickatall]

I'll say that right from the start. You have a problem with the actuator.

The actuator is a small stepper motor with a potentiometer that reports the motor position to the control unit. During calibration, this potentiometer is moved from one end to the other, and the control unit knows which position is valid at which resistance value. If this potentiometer starts to fail and there are positions where there is no longer any feedback, or where the resistance approaches infinity, the control unit loses its position and moves the flap to its end position. And then it gets warm.

Because the passenger side is causing the problem, the actuator that's also making the noise will be the problem.

This is just an educated guess, but I would replace the temperature actuator on the passenger side, even if it's a bit of work.

 

[OR VietVet]

We know the temp actuator, on the passenger side is failing, simply because it makes the ratcheting noise, during movement. That actuator's problem, was admitted to from the beginning. That does not explain how the compressor was shutting down and causing the whole system to blow warm air, when set to cold.

During the calibration process today, the left temp actuator, located above the transmission hump, was working fine and was blowing cold when, after the first calibration attempt I had cold on the left and warm on the right. Then a second calibration caused both sides to blow cold and then warm when commanded to.

I still believe that the first calibration did show the weakness of the right-p/s temp actuator and then it decided to work after a 2nd calibration attempt. BUT, I would bet she will experience a warm out of both and a compressor clutch not engaging, sometime down the road.

 

[nonickatall]

Well if one side suddenly blows warm, while the other keeps cold, it can definitely not be a problem with the compressor, refrigerant, relais or other reason with the cooling circuit.

It is a problem with actuator and/or control. Because it's clear due to the noises that's the actuator has a problem, it's likely that the problem comes from that.

I can imagine, but at least I don't know, that the control when the actuator is lost, goes in some kind of "limp mode" and switch the compressor off.

I would first replace the actuator and then look if the problem is gone. If not, I would change the control unit as well..

 

[OR VietVet]

Well if one side suddenly blows warm, while the other keeps cold, it can definitely not be a problem with the compressor, refrigerant, relais or other reason with the cooling circuit.

It is a problem with actuator and/or control. Because it's clear due to the noises that's the actuator has a problem, it's likely that the problem comes from that.

I can imagine, but at least I don't know, that the control when the actuator is lost, goes in some kind of "limp mode" and switch the compressor off.

I would first replace the actuator and then look if the problem is gone. If not, I would change the control unit as well..

Close to my thoughts. As far as I can tell and find during info searches and with input from my friend, a well known electrical specialist in the industry, there is no limp mode. I believe that with the strong possibility of the control head causing the intermittent shut down of the compressor clutch and easier to replace, we are going to likely do the control head replacement first. The cold on the left and warm on right was the very first time that happened and after 2nd calibration, did not happen again. She will monitor and report back. We have some 90+ temps the next few days and she will have plenty of times for using the a/c.

 

[Fless]

One thing you might look at is the calibration # (firmware) of the head unit. For many modules GM released updated calibrations after the initial units were built, so it could be that a subsequent calibration would correct this issue.

 

[OR VietVet]

Two days since calibration and the a/c still blows cold in 98 degree weather.